This invention relates to manufacturing graphite fibers by thermally decomposing natural gas. More particularly, this invention relates to growing the graphite fibers on a heavily oxidized surface of an iron-base alloy to improve the yield of product fibers per unit area.
U.S. Pat. No. 4,391,787, filed July 9, 1981 and assigned to the present assignee, describes a process for manufacturing thin, straight graphite fibers that are suitable for filler in plastic or other composites. In a preferred embodiment, the process comprises flowing a natural gas stream through a thin-wall stainless steel tube surrounded by wet hydrogen gas and heating to pyrolyze methane in the natural gas. Initially, no fiber growth is observed. During this time, the natural gas carburizes the steel and produces a tube inner surface that is believed to affect subsequent methane pyrolysis in a manner that induces fiber growth. Hydrogen diffusion through the tube from the surrounding gas is believed to promote formation of this surface. In any event, after several hours, graphite fibers grow on the inner wall pointing downstream. The fibers grow quickly in length up to several centimeters and thereafter principally grow radially. Pyrolysis is continued until the fibers are about 5 to 15 microns in diameter. In this process, the natural gas, the stainless steel and the wet hydrogen cooperate in growing the fibers at the elevated temperature, and each is believed to play an important role.
It is an object of this invention to provide an improved process for manufacturing graphite fibers by pyrolysis of a hydrocarbon gas. More particularly, it is an object of this invention to provide improvements in the above-described process for growing graphite fibers by natural gas pyrolysis, which increase the number of product graphite fibers per area of growing surface, and thus potentially the total fiber yield and the process efficiency, to thereby ultimately reduce the cost of the fibers. It is an object of one aspect of this invention to provide a process for growing profuse graphite fibers by natural gas pyrolysis adjacent stainless steel that does not require remote contact by any special gas, such as the wet hydrogen gas in the above process.
It is also an object of this invention to provide an improved method for growing graphite fibers by methane pyrolysis, which method grows the fibers on a surface that is particularly conducive thereto to obtain a high yield of product fibers per unit area. In a preferred embodiment, fibers are grown on a heavily oxidized surface of an iron-base alloy.
It is an object of another aspect of this invention to provide a pretreatment for preparing a surface of an iron-base alloy for growing graphite fibers thereon, which pretreatment increases the yield of product fibers per unit area. The pretreatment is particularly well suited for preparing selected regions of the iron-base surface, for example, regions from which fibers may be readily harvested.
It is a more specific object of this invention to provide a two-part iron-chromium alloy pyrolysis surface, wherein graphite fibers are grown on one part from a precursor formed by natural gas pyrolysis at the other part. The fiber-growing part initially bears a relatively heavy, predominantly iron oxide and forms numerous sites for nucleating fiber growth so as to produce a high number of fibers per unit area. The two-part surface may be employed in a natural gas pyrolysis process such as the one described above that employs wet hydrogen gas; however, it is surprising that profuse fibers may be grown even without wet hydrogen or other special gas in contact with the alloy.